There's a great thread on here about children and polyamory, but not one that specifically addresses what you're asking.
I have one child with my husband and another on the way. We had a gf who was a friend of the family for years (before we had the first child) whom I actually found very helpful and enlightening in her perspectives. But, while she was engaged in his life, and happy to offer advice, and he loved her dearly, she did NOT overstep her bounds and essentially tell us how to raise him. She took on an "aunt" role to him rather than third parent and it was a role we were ALL comfortable with. Should we ever form another poly relationship, I'd want to be with someone involved with the children as my family IS my life and inviting someone into my life and love means inviting them into my family.
It's really up to you and your husband to decide how much influence she should have in this situation. It's also up to her to decide how comfortable she is with your ultimate decision. Will she be a second mother, with all the adult privileges and responsibilities that go with? Will she take on a role of "aunt" or other loving and nurturing adult, but be expected to abide by the parenting standards you set down? Do you value her experience as a mother and have interest in her views, but maintain a "take it or leave it" approach? Is she expected to butt out completely when it comes to your child?
If you're bringing a baby into this world, and this lifestyle, these are things that need to be discussed between ALL of you BEFORE that baby is born. You also need to understand that, at least for awhile, this is going to create plenty of INEQUALITY as you redefine boundaries and needs and she may have different expectations about the raising of this child than you do. Your husband may even have different expectations. Even in mono couples parents often have at least a few strong opinions that differ on how children should be raised. These require communication and negotiation and compromise as it is, without throwing more opinions into the pot.
If you feel you have an equal relationship and want to keep it that way (in all areas except for the raising of the child) then you need to have a discussion between the three of you about your fears, hopes, desires, and expectations. You may find she's happy to step aside in this case. You may find your husband does or does not share your fears. Or she may not be able to handle the new unequal feel of the dynamic with you two being parents while she is not involved in that aspect. The point is, anything can happen. But, without first knowing everyone's feelings on it, all you can do is worry and guess. It isn't fair to bring a child into a relationship (of any dynamic) without being prepared for handling the needs of that child. Good luck.
I'd rather be hated for who I am than loved for who I am not.